The Synergistic Inhibitions of Tungstate and Molybdate Anions on Pitting Corrosion Initiation for Q235 Carbon Steel in Chloride Solution

In this work, the synergistic inhibitions of tungstate (WO(4)(2−)) and molybdate (MoO(4)(2−)) anions, including role and mechanism, on the initiation of pitting corrosion (PC) for Q235 carbon steel in chloride (Cl(−)) solution were investigated with electrochemical and surface techniques. The pitting potential (E(p)) of the Q235 carbon steel in WO(4)(2−) + MoO(4)(2-) + Cl(−) solution was more positive than that in WO(4)(2−) + Cl(−) or MoO(4)(2−) + Cl(−) solution; at each E(p), both peak potential and affected region of active pitting sites in WO(4)(2−) + MoO(4)(2−) + Cl(−) solution were smaller than those in WO(4)(2−) + Cl(−) or MoO(4)(2−) + Cl(−) solution. WO(4)(2−) and MoO(4)(2−) showed a synergistic role to inhibit the PC initiation of the Q235 carbon steel in Cl(−) solution, whose mechanism was mainly attributed to the influences of two anions on passive film. Besides iron oxides and iron hydroxides, the passive film of the Q235 carbon steel formed in WO(4)(2−) + Cl(−), MoO(4)(2−) + Cl(−), or WO(4)(2−) + MoO(4)(2−) + Cl(−) solution was also composed of FeWO(4) plus Fe(2)(WO(4))(3), Fe(2)(MoO(4))(3), or Fe(2)(WO(4))(3) plus Fe(2)(MoO(4))(3), respectively. The film resistance and the defect quantity for Fe(2)(WO(4))(3) plus Fe(2)(MoO(4))(3) film were larger and smaller than those for FeWO(4) plus Fe(2)(WO(4))(3) film and Fe(2)(MoO(4))(3) film, respectively; for the inhibition of PC initiation, Fe(2)(WO(4))(3) plus Fe(2)(MoO(4))(3) film provided better corrosion resistance to Q235 carbon steel than FeWO(4) plus Fe(2)(WO(4))(3) film and Fe(2)(MoO(4))(3) film did.